A polymer electrolyte fuel cell is a fuel cell in the form in which a polymer solid electrolyte is sandwiched between an anode and a cathode, a fuel is supplied to the anode, and oxygen or air is supplied to the cathode, whereby oxygen is reduced at the cathode to produce electricity. As the fuel, hydrogen, methanol, or the like is mainly used.
To enhance a reaction rate in a fuel cell and to enhance the energy conversion efficiency of the fuel cell, a layer containing a catalyst (hereinafter also referred to as a “fuel cell catalyst layer”) has been conventionally disposed on the surface of a cathode (air electrode) or the surface of an anode (fuel electrode) of the fuel cell.
As such a catalyst, noble metals have been generally used, and, among the noble metals, a noble metal stable at a high potential and having a high activity, such as platinum or palladium, has been mainly used. However, since these noble metals are expensive and limited in resource amount, development of substitutable catalysts has been desired.
Further, there has been a problem that the noble metals used on the surface of a cathode may be dissolved under an acidic atmosphere and are not suitable for uses requiring long-term durability. Accordingly, it has been strongly demanded that catalysts are developed which are not corroded under an acidic atmosphere and have excellent durability and high oxygen reducing ability.
As a catalyst substituted for noble metals, those entirely free of noble metals, such as base metal carbides, base metal oxides, base metal carbonitroxides, chalcogen compounds, and carbon catalysts, have been reported (for example, see Patent Literature 1 to Patent Literature 4). These materials are inexpensive and abundant in resource amounts as compared with noble metal materials such as platinum.
However, these catalysts containing base metal materials described in Patent Literature 1 and Patent Literature 2 have a problem that practically sufficient oxygen reducing ability is not obtained.
Further, the catalysts described in Patent Literature 3 and Patent Literature 4, although exhibiting high oxygen reduction catalytic activity, have a problem that stability under fuel cell operating conditions is extremely low.
As a catalyst substituted for noble metals, Nb and Ti carbonitroxides in Patent Literature 5 and Patent Literature 6 can effectively express the above-described performance and thus have received particular attention.
Although the catalysts described in Patent Literature 5 and Patent Literature 6 have extremely high performance as compared with conventional catalysts substituted for noble metals, a portion of the production step thereof needs to include heat treatment under a high temperature of 1600° C. to 1800° C. (for example, Example 1 of Patent Literature 5 or Example 1 of Patent Literature 6).
Such high-temperature heat treatment is not industrially impossible but involves difficulty and causes increase in equipment cost and difficulty in operation control, leading to increase in production cost, and, thus, the development of a method capable of inexpensive production has been desired.
Patent Literature 7 discloses a method for producing an electrode catalyst characterized by burning a mixed material of an oxide and a carbon material precursor but an electrode catalyst having sufficient catalytic performance has not been obtained.
Patent Literature 8 discloses a fuel cell electrode catalyst prepared by using a polynuclear complex of cobalt and the like but this catalyst has had problems that the toxicity of the raw material is high, a cost is high, and its catalytic activity is insufficient.
Further, since a metal element used in a catalyst substituted for a noble metal has been conventionally restrictive, it is desirable to be able to apply various metal elements to a catalyst substituted for a noble metal.